Manufacture of electrical discharge devices



Feb. 2, 1960 N. D. LEVIN MANUFACTURE OF ELECTRICAL DISCHARGE DEVICESFiled Feb. 27, 1959 A TTOR/VEY INVENTOR 72612/16272 17. l, evz'n mm mmmm an United States Patent MANUFACTURE OF ELECTRICAL DISCHARGE DEVICESNathan D. Levin, Skokie, 111., assignor to The Rauland Corporation, acorporation of Illinois Application February 27, 1959, Serial No.796,074 7 Claims. 01. 316-4) in and extend through an insulating pressmember bonded to the envelope. After the electrode elements are properlymounted, the press is sealed to the tube envelopeand the device is thenevacuated, gettered, and based according to well known techniques in theart.

However, when the getter, which is normally composed of such metal asMisch metal or barium is sputtered the first grid electrode and issupported and insulated Patented Feb. 2, 1960 use of such eyelets ishighly undesirable from the stand point of economy as they arerelatively expensive and involve several costly and time consuming stepsin order to complete the necessary welding operations, and the use ofslots has the disadvantage of weakening the press and alsoinvolvescostly construction techniques. 4

Another commonly encountered problem, especially in cathode-ray tubes,is electrical leakage between the cathode and the first grid of theelectrode assembly. The cathode is customarily mounted in telescopicrelation to from it by a transversely disposed ceramic wafer. Theinsulating wafer, having a flat surface of relatively large area, tendsto accumulate an electrically conductive sublimate in a manner similarto the accumulation on the inner surface of the press.

Therefore, one of the principal objects of the present invention is todevise a new and improved method of preventing or substantially reducingelectrical leakage between conductive elements, such as the lead-inwires or a pair of electrodes, of an electron-discharge deviceElectrical connections are made to the i during the gettering process, aportion of the getter matefial frequently settles onto the surface ofthe insulating press. This results in the formation of an electricallyconductive coating which substantially decreases the elec- .tricalleakage resistance across the surface of the press between the lead-inconductors and thus destroys the desirable insulating properties of thepress. In addition, during activation, and in fact, even during normaloperation of the finished tube, the electrodes and also the connectingstraps may emanate a metallic vapor when heated to their normaloperating temperature. Thus, through the process of sublimation, anelectric-ally conductive sublimate is again deposited on the innermostsurface of the press.

Various deterrents have been employed in the past to prevent formationof these leakage paths between the lead-in conductors to facilitatecontinued operation and thus obtain the maximum usefulness of the devicein operation. For example, it has been proposed to coat the press with agranular. insulating material whereby the effective surface area of thepress is increased several hundred times. In addition, each granule ofthe coating is shadow-casting and thereby prevents formation of acontinuous film of electrically conductive material between the lead-inconductors: The formation of such granular coatings to date, however,has necessitated the use of expensive materials and involvesseveralsteps of costly and time consuming manufacturing operations. Inaddition, the coatings alsotendto introduce deleterious impurities intothe tube.

The formation of such a continuous film of conductive material on thepress has also beenprevented by the use of eyelets which areindividually welded to the lead-in conductors and prov-ided'with flangesparallel to and closely spaced from the press surface; a third methodutilized the formation of a plurality of slots on the surface of thepress intermediate the lead-in conductors. However, the

in asimple, economical, yet highly effective manner.

.,A further object of the present invention is to devise a new andimproved method of manufacturing cathoderaytubes in which electricalleakage between the cathode and the first grid is prevented or greatlyreduced.

, Another. object of the present invention is to devise a ;new andimproved method for the purposes aforesaidw whereby deleteriousimpurities are not introduced into the device asan incident of theprocess.

A corollary object of the present invention is to devisesuch a-new andimproved method which is readily adaptable to mass productiontechniques,

,The present invention provides a new andimproved'v method for use inthe manufacture of an electron-dis charge device comprising, .within anenclosing envelope, a-. pair of conductive elements requiring theapplication of 1' different operating potentials, and an insulatingmember;- contacting both of the conductive elements. The inven tivemethod comprises the steps of coating the insulating: member with aliquid suspension of magnesium hydroxide: and heating the insulatingmember to a temperature sufficient to reduce the coating to an adherentgranular magnesium oxide insulating coating, whereby the forma tion ofan electrically conductive sublimation on the in sulating member betweenthe conductive elements, as an incidentto subsequent fabrication andoperation of' the device, is prevented. In actual practice, the heatapplied during the sealing of the press to the envelope or as anincident to'the evacuating operation, in which a baking cycle isnormally employed, is suflicient to reduce the magnesium hydroxide tomagnesium oxide, so that no additional heating step is required.

The features of the present invention which are believed to. be novelare set forth with particularity in the appended claims. The invention,together with further objects and advantages thereof, may best beunderstood by reference to the following description taken in connectionwith. the accompanying drawing, in the several figures of which likereference numerals identify like elements, and in which:.

Figure l is a cross-sectional view of a cathode-ray image-reproducerincorporating the present invention;

Figure 2 is a cross-sectional view taken along the line 2-2 of Figure l;I I Figure 3 is a fragmentary cross-sectional view taken along theline3--3 of'Figure 2; q t

Figure 4 is an enlarged fragmentary cross-sectional viewdiagrammatically illustrating the details of the granular coating; and

.-Figure 5 is an enlarged fragmentary cross-sectional view showing theinvention as applied to reduce electrical I 2,923,585 I I t leakagebetween the cathodeand first grid electrode of the device of Figure 1.

With reference to Figure; 1, there is shown, for illustrative purposesonly a typical cathode-ray image-reproducer incorporating the presentinvention. The. particular reproducer show-n comprises anenclosing'glassenvelope having a bulb-portion lh, a rearwardlyprojecting neck portion 11-, and a fluorescent screen 12 deposited ontheinnermost surface of faceplate 13. Aconventional electron gunassembly'is-mounted.Within neck'portion 11 and "comprises essentiallyan-aluminum oxide coated filament-ldmountedwithinanickel cathode sleeve15. A carbonate'coating 16, composed of barium carbonate and asuitablebinder added thereto or a mixture of barium carbonate with thecarbonates of other alkaline-earth metals, issprayed or otherwiseapplied to the exposed face of cathode sleeve '15 and is renderedthermionically emissive when reduced to an oxide. An aperturedcontrolelectrode 17 is insulated from cathode byceramic insulating wafer 9 andcompletely encompasses cathode 15 with the aperture thereof in closeproximity to coating 16.

The electron gun further includes apertured electronbeam forming anodes18 and 19 followed by beam focusing anodes 20 and'21. A finalaccelerating anode 22, consisting of a colloidal graphite coatingsprayed or otherwise formed on the innermost surface of the envelope, iselectrically connectedto anode 21 by contact spring members 23 mountedthereon in contact with coating 22. A getter strap 24 is afiixed toanodeZl and supports a plurality ofglobules of barium getter material 25which is sputtered by the application of an external magnetic field togenerate eddy currents in strap 24, as is well known in the.a rt. Theelectrode elements are secured in a fixed position operatively relatedto one another by metallic straps 26 welded thereto and heat-fused toglass support rods 27." Electrical connections are made to theelectrodes by aplurality of lead-in wires 28 respectively'welded theretoand which are sealed in and extend through an insulating press member 29which, in turn, is fused to the end of neck 11. The lead-in wires arecut to proper lengths and respectively welded to a plurality of pins 30mounted on a phenolic base member 31 secured to neck portion 11 andcompletely enclosing leadin wires 28 and press 29.

After the tube elements are properly mounted, the envelope is evacuatedto a pressure of approximately 3X10 millimetersof mercury. Analternating voltage of approximately four volts (R.M.S.) is applied tofilament 14 for approximately fifteen seconds to heat cathode, 15 to atemperature of approximately 680 C. At this point, the cathode coatingbinder material is driven off and the voltage applied to filament 14 isincreased to approximately eight volts to .raisethe temperature ofcathode 15 toapproximately 875 C. After a time lapse of approximatelyfifteen seconds, reduction of carbonate coating,16 to an oxide begins,after which, the voltage applied to filament 14 is raised to tenvoltstoincrease the temperature of cathode 15 to approximately 950 C. Afterapproxi mat'ely thirty seconds, the reduction is completed and withcontinued heating, coating 1 6 is rendered thermionically emissive;thereafter the tube is gettered to remove any residual contaminatinggases and is based according to principles well known in the art.

It, is to be appreciated, of course, that the exact details of thisactivation process are entirely conventional and may vary greatly inactualcommercial production.

As before mentioned, however, during the activation, ageing. andgettering processes and also during normal operation of the device, anelectrically conductive coating shown as 32, is deposited on theinnermost surface of; press 29 and wafer 9 and thereby greatly,increases the electrical leakage between lead Wires 28 and betweencathode. 15 and control electrode 17. However, before the, envelope. isevacuated and in accordance with the present invention, press 29 andwafer 9 are coated by brushing, spraying or the like with a finelydivided insoluble suspension of magnesium hydroxide, commonly known asmilk of magnesia. In order to prevent the introduction of water vaporand other deleterious impurities into the tube envelope duringprocessing, it is preferred that the magnesium hydroxide coating eitherbe dried or allowed to dry before the press and electron gun assembly issealed to the neck. During the sealing-in operation in which heat isapplied to fuse press 29 to the neck of the tube envelope, themagnesiumhydroxide coating is simultaneously reduced to form a granular adherentinsulating coating of magnesium oxide on the press as :showndiagrammatically'as 33 in Figures 3 and 4. Sub- :sequent processing ofthe device during the exhaust cycle involves the application ofsufiicient heat to reduce the magnesium hydroxide coating on ceramicwafer 9 to a granular adherent insulating coating 35 of magnesium oxide,as shown in Figure 5. t

' With reference to Figure 4, if it is assumed, for illustrativepurposes only, that the entire source of metallic 'vapor is remotelylocated at point 34, and as it is well known that the metallic vaporwill be deposited on coat- .ing 33. along straight line projections, itcan be seen that, in addition to effectively increasing the surface areaof the press, each granule of coating 33 is shadow-casting andconsequently prevents the formation of a continuous :film ofelectrically conductive material, thereby preserving the desirablehighelectrical potential characteristics of press 29; Even though theentire source of metallic vapor is notin' reality remotely located at asingle point, coating-'33 neverthelessprovides sutficient shadowingeffeet to prevent the formation of a continuous conductive film on thepress. Similarly, granular coating 35 on innlating spacer 9 preventsconductive vapor from cathode 15 or its emissive surface 16 from forminga continuous film on the surface of spacer 9.

The use of magnesium hydroxide as a coating material has severaldistinct advantages over the various expedients utilized in thepast;First of all magnesium hydroxide is relatively inexpensive andcommercially available. It does not necessitate any special preparationsnor the exercise of special cautions before or during application andmay be used directly as purchased. In addition, neither magnesiumhydroxide nor magnesium oxide introduces deleterious. impurities intothe tube either during processing or even during operation of the deviceand is readily adaptable to application by automatic techniques.

Therefore it is quite apparent that in accordance with the presentinvention, there'has been provided a new and improved method ofsubstantially preventing electri cal leakage along an insulating memberextending between conductive elements of an electron-discharge'device ina simple, economical, yet highly effective manner, and the method isreadily adaptable to mass production techniques thereby maintaining thecost of the finished product at a minimum.

While a particular embodiment of the invention has been shown anddescribed, it will be obvious to those skilled in the art that changesand modifications may be operating-potentials, and an insulating membercontacting both; of said conductive elements, the method ofsubstantially preventingelectrical leakage between said conductiveelements comprising the steps of: coating said insulating member with aliquid suspension of mageSium hydroxide; and heating said insulatingmember to aternper ature sufiicient to reduce said coating: to anadherent granular magnesium oxide insulating coating,

, whereby the formation of a continuous film of an electricallyconductive sublimate on said insulating member between said conductiveelements, as an incident to subsequent fabrication and operation of saiddevice, is prevented.

2. In the manufacture of an electron-discharge device comprising anenclosing envelope, a press sealed to said envelope, a plurality oflead-in wires sealed in said press, and a plurality of electrodesrespectively connected to said lead-in wires, the method ofsubstantially preventing electrical leakage between said lead-in wirescomprising the steps of: coating said press with a liquid suspension ofmagnesium hydroxide; and heating said press to a temperature sufficientto reduce said press coating to an adherent granular magnesium oxideinsulating coating, 'w-hereby the formation of a continuous film of anelectrically conductive sublimate between said lead-in-wires, as i anincident to subsequent fabrication and'operation of said device, isprevented.

3. In the manufacture of an electron-discharge device comprising anenclosing envelope, a press sealed to said envelope, a plurality oflead-in wires sealed in said press, and a plurality of electrodesrespectively connected to said lead-in wires, the method ofsubstantially preventing electrical leakage between said lead-in wirescomprising the steps of: coating said press with a liquid suspension ofmagnesium hydroxide; drying said coating; and thereafter heating saidpress to a temperature sufiicient to reduce said press coating to anadherent granular magnesium oxide insulating coating, whereby theformation of a continuous film of an electrically conductive sublimatebetween said lead-in wires, as an incident to subsequent fabrication andoperation of said device, is prevented.

4. In the manufacture of an electron-discharge device comprising anenclosing glass envelope, a glass press sealed to said envelope, aplurality of lead-in wires sealed in said press, and a plurality ofelectrodes respectively connected to said lead-in wires, the method ofsubstantially preventing electrical leakage between said lead-in wirescomprising the steps of: coating said press with a liquid suspension ofmagnesium hydroxide; placing said press in contact with said envelope;and heating the envelope and press throughout the mutually contactingsurfaces thereof to effect fusion therebetween and to simultaneouslyreduce said press coating to an adherent granular magnesium oxideinsulating coating, whereby the formation of a continuous film ofelectrically conductive sublimate between said lead-in wires, as anincident to subsequent fabrication and operation of said device, isprevented.

5. In the manufacture of an electron-discharge device comprising withinan enclosing envelope, a tubular control grid, a cathode, and meansincluding an insulating spacer for supporting said cathode within saidtubular control grid, the method of substantially preventing electricalleakage between said cathode and said control grid comprising the stepsof: coating said insulating spacer with a liquid suspension of magnesiumhydroxide; and heating said spacer to a temperature sufficient to reducesaid coating to an adherent granular magnesium oxide insulating coating,whereby the formation of a continuous film of an electrically conductivesublimate on the surface of said spacer between said cathode and saidcontrol grid, as an incident to subsequent fabrication and operation ofsaid device, is prevented.

6. In the manufacture of an electron-discharge device comprising withinan enclosing envelope, a tubular control grid, a cathode, and meansincluding an insulating spacer for supporting said cathode within saidtubular control grid, the method of substantially preventing electricalleakage between said cathode and said control grid comprising the stepsof: coating said insulating spacer with a liquid suspension of magnesiumhydroxide; drying said coating; and thereafter heating said spacer to atemperature sufiicient to reduce said coating to an adherent granularmagnesium oxide insulating coating, whereby the formation of acontinuous film of an electrically conductive sublimate on the surfaceof said spacer between said cathode and said control grid, as anincident to subsequent fabrication and operation of said device, isprevented.

7. In the manufacture of an electron-discharge device comprising anenclosing glass envelope and an electrode system including a tubulargrid electrode, a cathode, and

means including an insulating spacer for supporting said cathode withinsaid tubular grid electrode, the method of substantially preventingelectrical leakage between said cathode and said grid electrodecomprising the steps of: coating said insulating spacer with a liquidsuspension of magnesium hydroxide; assembling said electrode systemincluding said coated spacer inside said envelope; and heating theenvelope and said electrode system while evacuating said envelope tosimultaneously reduce said press coating to an adherent granularmagnesium oxide insulating coating, whereby the formation of acontinuous film of electrically conductive sublimate on said insulatingspacer between said cathode :and said control grid, as an incident tosubsequent fabrication and operation of said device, is prevented.

References Cited in the file of this patent UNITED STATES PATENTS

